Method for fabricating a single-crystal semiconductor

ABSTRACT

An apparatus and a method for fabricating a single-crystal semiconductor by means of CZ method are provided for improving the quality control through the modification of thermal cycle of a pulled single-crystal semiconductor. The apparatus includes a ring after heater which is capable of elevation. The method decreases a temperature gradient to smaller than 20° C./cm, and preferably under 15° C./cm, when the pulled single-crystal semiconductor is cooled from 1200° C. to 1000° C. The after heater therefore heats the single-crystal semiconductor where there is a temperature of 100-300° C. lower than the range of 1200-1000° C. A thermal shelter is provided to retain a temperature gradient of larger than 20° C./cm when the single-crystal semiconductor is within the temperature range between the melting point and 1250° C. The after heater and the shelter can be raised to an upper portion when polysilicon blocks are charged and a twisting step is carried out.

This application is a Division of application Ser. No. 08/801,282 filedFeb. 18, 1997 now U.S. Pat. No. 5,853,480.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and a method forfabricating a single-crystal semiconductor by means of Czochralskimethod, and more specifically, for fabricating a single-crystalsemiconductor in which the precipitated oxygen ions and thermal donorsare uniformly distributed, thereby reducing the defects in an as-grownsingle-crystal semiconductor and improving the strength of an oxidefilm.

2. Description of Related Art

Semiconductor devices are generally formed on a high-puritysingle-crystal silicon wafer which is, in general, fabricated byCzochralski (CZ) method. The CZ method charges polysilicon blocks in acrucible inside a single-crystal fabricating apparatus and melts theblocks by heaters around the crucible. A seed which is held by a seedholder is then immersed in the melt and lifted with rotating inclockwise or counter-clockwise direction to grow the single-crystalsilicon.

The quality characteristics of the grown single-crystal silicon, such asthe strength of an oxide film, the amount of oxygen precipitation, andthe bulk defects, depend on the heat cycle in the CZ fabricationprocesses. The heat cycle has therefore been improved in certain relatedarticles. For example, Japanese Laid Open No. 6-211591 and No. 6-211592disclosed a method for fabricating a single-crystal silicon rod whichhas uniform precipitated oxygen ions and a firm oxide film. The methodutilizes a ring heater whose power is supplied by a heat controller, andwhich is supported by an electrode, thereby gradually cooling the liftedsingle-crystal silicon. Moreover, the heat cycle can be modified byadjusting the power supplied to the ring heater.

However, the arrangement of the ring heater causes the followingdisadvantages.

1. The process cannot immediately response to the degradation of thegraphite crucible and the variation lifting conditions such as theposition of the melt surface and the condition of introduced gas.

2. In order to prevent the influence to the heater, the crucible must belowered down when the polysilicon blocks are charged therein. The meltconditions are therefore affected.

3. Since the ring heater screens the seed, the twisting step forpreventing stacking faults prior to the growth of the single-crystalcannot be observed.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides an apparatus and a methodfor fabricating a single-crystal semiconductor in which the qualitycharacteristics, such as the oxide film strength and the distributionsof precipitated oxygen and bulk defects, are improved.

The present invention response immediately to the environmentalvariation inside a furnace, thereby melting polysilicon blocksefficiently and facilitating the twisting step for preventing stackingfaults.

The apparatus of the invention, which fabricates a single-crystalsemiconductor by means of CZ method, includes an after heater around thesingle-crystal semiconductor; an elevator for elevating the afterheater; and a controller for controlling the electric power supplied tothe after heater and the elevator.

The method of the invention fabricates the single-crystal semiconductorby means of the aforementioned apparatus. In the method, a temperaturegradient changes in a specific region where the pulled single-crystalsemiconductor goes through, and the after heater is located where thereis a temperature of about 100-300° C. lower than the temperature of thespecific region, thereby heating the pulled single-crystalsemiconductor.

The method is characterized in that a temperature gradient of largerthan 20° C./cm exists in the pulled single-crystal semiconductor whosetemperature is reduced from the melting point to 1250° C., and anothertemperature gradient of smaller than 20° C./cm, preferably under 15C/cm, exists where the temperature is reduced from 1200° C. to 1000° C.

Moreover, the invention raises the after heater to the upper portion ofa furnace when polysilicon blocks are charged in a crucible as well as atwisting step is carried out to eliminate stacking faults, and lowersthe after heater to a predetermined position to heat a specific regionof the single-crystal semiconductor when the single-crystalsemiconductor is pulled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the apparatus for fabricating asingle-crystal semiconductor according to a first embodiment of theinvention;

FIG. 2 is a schematic diagram illustrating that an after heater and aheat shelter of the apparatus of FIG. 1 are elevated;

FIG. 3 is a schematic diagram illustrating the apparatus of a secondembodiment of the invention;

FIG. 4 is a schematic diagram illustrating the apparatus of a thirdembodiment of the invention;

FIG. 5 is a schematic diagram illustrating the apparatus of a fourthembodiment of the invention; and

FIG. 6 illustrates an alternative configuration of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides an apparatus and a method for improving thethermal cycle in fabrication of a single-crystal semiconductor by meansof CZ method. The apparatus includes an after heater and a controllerfor controlling the location and input power of the after heater,thereby heating a predetermined position of the growing single-crystalsemiconductor and reducing thermal gradient thereof. Moreover, theapparatus responses immediately to the degradation of the graphitefurnace and environmental variation inside the furnace, such as thevariation of pulling conditions, by modifying the location of the afterheater and its output power.

The method of the invention provides an after heater around the growingsingle-crystal semiconductor where the temperature is cooled to about100-300° C., thereby reducing the temperature gradient thereof andimproving the quality characteristics related to the thermal cycle, suchas the oxide film strength, amount of oxygen precipitation, and bulkdefects.

The formation of defects in an as-grown single-crystal silicon dependson the numbers of lattices and voids therein. When the single-crystalsilicon has a temperature of about 1180° C., the dimensions and densityof the defects are determined. The invention therefore smoothes thetemperature gradient of the single-crystal silicon when the temperatureis reduced from 1200° C. to 1000° C. The region where the temperature isbetween 1100° C. and 700° C. is heated by the after heater, whereas theregion near the melt surface, having a temperature of between themelting point and 1250° C., has larger temperature gradient, therebyfacilitating the growth of the single-crystal. Therefore, the quality ofthe single-crystal silicon is improving without affecting theproductivity.

The after heater can be drawn back to the upper portion of the furnace.Therefore the polysilicon blocks are charged and melted steadily, andthe twisting step for prevention stacking faults can be easily observed.Moreover, the location and temperature of the after heater can beprogrammed in advance for performing precise thermal cycle forsingle-crystal silicon rod of any length.

The embodiments of the invention will be described in accompaniment withthe drawings. Referring to FIG. 1, the apparatus of a first embodimenthas a main chamber 1. In the main chamber 1, there are a quartz crucible3 for reserving single-crystal silicon material and a graphite crucible4 for supporting the quartz crucible 3. The graphite crucible 4, whichis capable of rotation and elevation, is arranged on a crucible spindle5. Around the graphite crucible 4 are provided with a ring heater 6 anda thermal preserver 7.

The upper portion of the main chamber 1 is connected to a pull chamber10 through a connecting chamber 8 and a upper chamber 9. A seed holder12 is appended to a wire 11 which is pulled by a wire driver (not shownin the figure). When a seed held by the seed holder 12 is melted in themelt, the seed holder 12 is rotated in clockwise or counter-clockwisedirection according to the graphite crucible 4, and is lifted up to thepull chamber 10. Since the connecting chamber 8 is provided foraccepting an after heater and thermal shelter, the main chamber 1 candirectly connect to the pull chamber 9 if the main chamber 1 is highenough.

The exterior of the pull chamber 10 has an elevator 14 and two shaft 16which can be elevated along a guide rod 15 and are supported by asupporter 17. Another supporter 18 is attachable and removable withrespect to the supporter 17. The lower end of the supporter 18 isconnected to an after heater 19 which is arranged around thesingle-crystal silicon 13. The shaft 16, supporters 17 and 18, and theafter heater 19 are all made of graphite. As controlled by an electricalcontroller (not shown in the figure) and through the shaft 16 andsupporters 17 and 18, the high frequency power or hot resistance of theafter heater 19 heats a specific region of the single-crystal silicon toa predetermined temperature. Other elevators (not shown in the figure)can be arranged around the after heater 19 for elevating a cone-shapedthermal shelter 20.

The method according to the embodiment of the invention will bedescribed. Referring to FIG. 2, the elevator 14 is driven to raise theafter heater 10 when the polysilicon blocks are charged in the quartzcrucible. The thermal shelter 20 is then also elevated by another notshown elevator. Therefore, a space is formed over the crucible 3 forpreventing the interference between the polysilicon material 22, theafter heater 19 and the thermal shelter 20. Moreover, since the crucibleis located where the thermal efficiency is very high, the polysiliconmaterial can be melted rapidly.

The non-illustrated wire driver is driven to immerse the seed in themelt when the polysilicon blocks 22 are completely melted in thecrucible 3. The wire which connects the seed is then gradually pulled upfor performing the twisting step for eliminating stacking faults. Sincethe after heater 19 and thermal shelter 20 has been drawn back in theupper portion, the process of the twisting step can be clearly observed.The after heater 19 and the thermal shelter 20 are then dropped to theirpredetermined position after the twisting step. For example, the afterheater 19 is located around the growing single-crystal silicon where thetemperature is between 800-700° C. The seed holder 12 is lifted to thepull chamber 10 for the growth of the single-crystal silicon 13. Athermometer (not shown in the figure) is provided for detecting thetemperature of the after heater 19 which is sent back to the electricalcontroller, thereby retaining the after heater 19 in a constanttemperature. The temperature of the single-crystal silicon thereforereduces gradually from 1200° C. to 1000° C. as the after heater 19 heatsa specific region thereof.

Moreover, the thermal cycle which is determined by the temperature andlocation of the after heater 19 can be programmed in an electricalcontroller (not shown). Therefore, the settings of the program areadjustable as the pull length of the single-crystal silicon varies.

Various examples for fabricating the single-crystal silicon according tothe aforementioned method will be described. In these examples, theoutput power of the after heater is 5 kW or 7 kW. The after heater canbe arranged at five positions which correspond to five temperaturegrades between 1100° C. and 400° C. Moreover, a conventional apparatuswithout after heater is also utilized as reference. The outcomes of theexamples, such as the temperature gradient, yield and difficulty inlifting the single-crystal silicon, are shown in Table 1.

As shown in Table 1, the yield is improved by locating the after heaternear the region where the crystal temperature is about 800° C. or 700°C., no matter the output power is 5 kW or 7 kW. Therefore, the pull ofthe single-crystal silicon becomes easier. When the after heater islocated near the region having a temperature of 1100 or 1000° C., sincea temperature gradient of smaller than 20° C./cm exists in the rapidcooling region between the melting point and 1200° C., and a temperaturegradient of larger than 15 C/cm exists in the gradual cooling regionbetween 1200° C. and 1000° C., the yield is reduced to under 60%. Thediameter of the lifted single-crystal silicon is therefore difficult tocontrol. Similar situation happens when the after heater is located nearthe region having a temperature of 400° C. The reference case withoutafter heater has a temperature of more than 25 C/cm in the gradualcooling region between 1200° C. and 1000° C. The yield is thereforereduced to 50%.

The examples illustrate that the after heater is preferably locatedwhere there is a crystal temperature of about 800° C. or 700° C. Thetemperature gradient in the gradual cooling region between 1200° C. and1000° C. is then under 15 C/cm, thereby improving the thermal cycle ofthe single-crystal silicon.

FIG. 3 illustrates a portion of the apparatus for fabricating asingle-crystal semiconductor according to a second embodiment of theinvention. Referring to FIG. 3, the upper portion of the upper chamber 9is provided with a short thermal shelter 23 with a downward orientation.Since the after heater 19 can be raised to the lower end of the thermalshelter 23, each region of the pulled single-crystal silicon can begradually cooled.

FIG. 4 illustrates a portion of the apparatus for fabricating asingle-crystal semiconductor according to a third embodiment of theinvention. Referring to FIG. 4, the upper portion of the upper chamber 9is provided with a short thermal shelter 24 with a downward orientation.The thermal shelter 24, similar to the thermal shelter 20, can reach thesurface region of the melt 2. The thermal shelter 24 is capable ofisolating the thermal radiation in the main chamber 1 and that in theupper chamber 9. Therefore, the specific region of the single-crystalsilicon can be gradually cooled by the after heater 19 even though thegrowing region must be rapidly cooled. The after heater 19 is movablealong the inner side of the thermal shelter 24.

FIG. 5 illustrates a portion of the apparatus for fabricating asingle-crystal semiconductor according to a fourth embodiment of theinvention. Referring to FIG. 5, the apparatus has the after heater 19but no thermal shelter. The after heater 19 is capable of elevation,thereby gradually cooling a desirable region of the pulledsingle-crystal silicon 13.

The second, third and fourth embodiments of the invention, similar tothe first embodiment, cool the specific region of the pulledsingle-crystal silicon, thereby providing the desirable thermal cycle.Moreover, in these embodiments, the electrical controller which controlsthe after heater is the same as that of the first embodiment.

The apparatus for fabricating the single-crystal silicon not onlyimproves the thermal cycle but increases the amount of polysiliconmaterial charged in the crucible. For example, referring to FIG. 6, whenthe polysilicon blocks in the quartz crucible 3 have been melted, apolysilicon rod 25 appending to the wire 11 can be heated by the afterheater 19, thereby decreasing the heavy load of the quartz crucible 3.

The invention therefore has the following advantages.

1. The apparatus and method for fabricating the single-crystalsemiconductor by means of CZ method utilize the after heater to smooththe temperature gradient of the crystal in a temperature range which isvery important to the quality of the single-crystal semiconductor.Therefore, the high quality single-crystal semiconductor which has highoxide film strength, low amount of oxygen precipitation and bulk defectscan be easily fabricated by modifying the thermal cycle to a desirableorientation.

2. Since the after heater is capable of elevation, and its temperatureis controllable, the thermal cycle of the single-crystal silicon can beprecisely controlled in a wide range.

3. The after heater can be drawn back to facilitate the processes suchas melting of polysilicon blocks and the twisting for eliminatingstacking faults.

4. The apparatus of the invention can be established by modifying aconventional one, thereby reducing the manufacturing cost.

                                      TABLE 1                                     __________________________________________________________________________                Output of oiler heater                                                        5k W                  7k W                  no after              __________________________________________________________________________                                                            heater                Crystal temperature of                                                                    1100° C.                                                                    1000° C.                                                                    800° C.                                                                    700° C.                                                                    400° C.                                                                    1100° C.                                                                    1000° C.                                                                    800° C.                                                                    700° C.                                                                    400° C.                                                                    --                    where the after h heder                                                       is located                                                                    temperature gradient                                                          ° C./cm                                                                melting point                                                                             17.5 19.2 25.5                                                                              28.7                                                                              29.0                                                                              15.0 16.8 23.2                                                                              26.2                                                                              28.0                                                                              32.1                  ˜1250° C.                                                        1200° C.˜1000° C.                                                     18.2 16.8 13.3                                                                              11.4                                                                              16.8                                                                              17.9 16.6 11.1                                                                              10.0                                                                              16.8                                                                              25.7                  yield (%)   55   56   70  80  55  55   58   75  88  56  50                    lift difficulty                                                                           X    Δ                                                                            ◯                                                                     ⊚                                                                  ⊚                                                                  X    Δ                                                                            ◯                                                                     ⊚                                                                  ⊚                                                                  ⊚      __________________________________________________________________________     (note:                                                                        ⊚: very easy                                                   ◯: easy                                                           Δ: difficult                                                            X: can't control the diameter)                                           

What is claimed is:
 1. A method for fabricating a single-crystalsemiconductor by means of an apparatus comprising:an after heater aroundthe single-crystal semiconductor; an elevator for elevating the afterheater; and a controller for controlling the electric power supplied tothe after heater and the elevator, wherein a temperature gradientchanges in a specific region where the growing single-crystalsemiconductor goes through, and the after heater is located where thereis a temperature of about 100-300° C. lower than the temperature of thespecific region, thereby heating the pulled single-crystalsemiconductor.
 2. The method as claimed in claim 1, wherein atemperature gradient of larger than 20° C./cm exists in the pulledsingle-crystal semiconductor whose temperature is reduced from themelting point to 1250° C., and another temperature gradient of smallerthan 20° C./cm, preferably under 15 C/cm, exists where the temperatureis reduced from 1200° C. to 1000° C.
 3. A method for fabricating asingle-crystal semiconductor raising an after heater to the upperportion of a furnace when polysilicon blocks are charged in a crucibleand a twisting step is carried out to eliminate stacking faults, andlowering the after heater to a predetermined position to heat a specificregion of the single-crystal semiconductor when the single-crystalsemiconductor is pulled.